(1) Field of the Invention
This invention relates to variable frequency oscillator, the frequency of which is controllable, particularly a complementary MOS oscillator.
(2) Description of the Related Art
Voltage controlled oscillator (VCO) is widely used in the phase locked loop of a frequency synthesizer. In such an application, the frequency of the VCO must lock in with a reference frequency, which is customarily generated from a crystal oscillator. In such a system, the crystal frequency is compared with the free running frequency of the VCO. When the two frequencies are not in lock, an error signal is generated. This error signal is used to change the frequency of the VCO until the two frequencies are locked.
A widely used element used in the variable frequency oscillator is a varactor. The varactor is a reverse-biased diode and the junction capacitance varies according to the voltage applied. A varactor diode is similar to the emitter junction of a bipolar transistor. In a CMOS structure, an emitter like junction is not available. To incorporate such a junction requires extra processing and area, hence is more costly.
A requirement for the phase locked loop to operate satisfactorily is that the free running frequency of the VCO must be within the lock range of the reference frequency. If the free running frequency is too far off the reference frequency and outside the lock range, the phase-locked loop may not be able to lock in with the reference frequency. The free running frequency of the VCO is effected by the operating temperature of the phase locked loop. VCO may not be able to lock in the reference frequency as temperature changes.